The present invention generally relates to electronic ballasts, and more particularly to a method and circuit for controlling the current in a high pressure discharge lamp.
In starting a high intensity discharge (HID) lamp, the lamp experiences three phases before achieving steady-state operation. These phases include breakdown, glow discharge, and thermionic emission. Breakdown requires a high voltage to be applied to the lamp. Following breakdown, the voltage must be high enough to sustain a glow discharge and heat the electrode to thermionic emission. Once thermionic emission commences, current must be maintained, in the run-up phase, until the electrode reaches its steady-state temperature. Upon completion of the run-up phase, the lamp can be operated with a lower level of current in the steady-state operating mode.
Lamp manufacturers generally specify that, during the run-up phase, the lamp current needs to be limited to less than 2X, where X is the steady-state rated lamp current. If the current is not so limited during run-up, the electrodes of the lamp may be damaged and/or the useful life of the lamp may be reduced. Moreover, it is desirable that the current during run-up be selectively controlled so as to achieve steady-state operation efficiently and promptly. Accordingly, it is desirable to provide an electronic ballast which selectively controls the current during run-up.
One known type of low frequency ballast employs a buck converter and a full-bridge inverter. The buck converter functions as a current source and provides a magnitude limited current to the lamp. The full-bridge inverter ensures that the current through the lamp is reversed in a periodic manner.
A conventional buck converter does not allow for selective control of the lamp current during the run-up phase. One known prior art solution is to modify the buck converter so that it operates at a variable frequency during the run-up phase (as compared with a fixed frequency during steady-state operation). Another solution, currently unknown in the prior art, is to operate the buck converter at a variable duty cycle during the run-up phase.
Accordingly, it is desirable to provide a control circuit that operates a buck converter at a variable duty cycle during the run-up phase so as to preserve the useful life of the lamp and promptly achieve steady-state operation.